U.S. Pat. No. 3,711,760 relates to a power supply system for supplying AC power to a DC load via a rectifier system.
In the power supply system suggested in U.S. Pat. No. 3,711,760 a three phase AC power source supplies AC power to two six phase transformers each having a primary, a secondary and a tertiary winding. The primary windings are directly connected to the AC power source. The secondary windings are operatively connected to a twelve phase rectifier which supplies the DC load with DC power. The windings of each of the two six phase transformers are arranged in such a manner that the fifth, seventh, seventeenth and nineteenth harmonics cancel each other.
The tertiary windings are connected to harmonic filters so that the eleventh, thirteenth, twenty-third and twenty-fifth harmonics are suppressed by said harmonic filters.
The power supply system of U.S. Pat. No. 3,711,760 is intended for supplying large amounts of DC power to a DC load.
Within the field of wind energy large amounts of AC power is to be delivered from individual wind turbines or wind turbine plants to AC distribution networks. Since wind turbines are often operated at variable speeds the frequency of the generated power varies in accordance with the variable frequency. For that reason AC/AC converters having an intermediate circuit are applied to keep the frequency of the AC power to be injected into an AC distribution network within the limits set by the grid codes of the particular network.
AC/AC converters of wind turbine systems generate unwanted voltage or current harmonics. These unwanted voltage or current harmonics should be prevented from reaching a power supply grid operatively connected to the wind turbine system. As step-up transformers are normally required between AC/AC converters and associated power supply grids, it is of interest to make use of the transformer reactance. It is well-known, cf. FIG. 1, to use a step-up transformer as a phase reactor and to apply tuned LC-branches on the high voltage side of the step-up transformer. This removes the need for the separate phase reactor, but has two disadvantages: (i) a dv/dt filter (which is physically small) must be inserted between AC/AC converter and step-up transformer, and (ii) the LC-branch is connected at high voltage level.
It may be seen as an object of embodiments of the present invention to provide a simple solution for suppressing harmonics in wind power systems.
It may be seen as a further object of embodiments of the present invention to provide a solution where harmonic suppression is performed at a low voltage level.